Optical sensor

ABSTRACT

A light detection device 1 comprises a photocathode 30 and an electron multiplier 50, which are positioned inside a vacuum container 10. A photomultiplier tube is arranged from these components. Light detection device 1 is equipped with an optical fiber 20, through which a light signal L flows, and photocathode 30 is formed on an end face 27 of optical fiber 20.

TECHNICAL FIELD

This invention concerns a light detection device that includes anoptical part, such as a photomultiplier tube.

BACKGROUND ART

FIG. 3 is a schematic view of a prior-art light detection device. Thisprior-art light detection device includes a photomultiplier tube 80 andan image forming system 90. Photomultiplier tube 80 has a structure,wherein an electrode 83 a, a photocathode 85, an aperture electrode 83b, a focusing electrode 83 c, an electron multiplier 87, and a readoutelectrode 83 d are positioned inside a vacuum container 81 in that orderfrom one end face to the other end face of vacuum container 81. Imageforming system 90 comprises lens systems 91 and 93, positioned so as tooppose each other, a wavelength selection filter 95, positioned betweenlens system 91 and lens system 93, and an adjustment part 97 for fineadjustment of the position of lens system 93. The necessary wavelengthcomponent within a light signal L is selected by wavelength selectionfilter 95.

Light signal L from a light source S is imaged onto photocathode 85 byimage forming system 90. By fine adjustment of lens system 93 usingadjustment part 97, the adjustment of the imaging is performed. By thisimaging, electrons inside photocathode 85 are excited and photoelectronsare emitted into the vacuum (external photoemission effect). Of thephotoelectrons that are emitted, the photoelectrons that pass through anopening 82 of aperture electrode 83 b are focused on electron multiplier87 by focusing electrode 83 c. By secondary electron emission occurringrepeatedly at electron multiplier 87, the electric current is amplified.This is read out as the output signal via readout electrode 83 d.

With the above-described light detection device, when the intensity oflight signal L that is made incident on photocathode 85 is extremelylow, the signal-to-noise ratio in measurement is strongly affected bythermal noise. That is, as thermal noise increases, the signal-to-noiseratio in measurement worsens. It is thus important to reduce the thermalnoise. The thermal noise can be reduced by lowering the temperature ofphotocathode 85 and by making the area of photocathode 85 small. Inprior arts, the temperature of photocathode 85 is lowered by positioninga Peltier cooler 89 in the vicinity of photocathode 85 or by reducingthe effective area of photocathode 85 by means of aperture electrode 83b. The area corresponding to the opening area of opening 82 of apertureelectrode 83 b corresponds to being the effective area of photocathode85.

DISCLOSURE OF THE INVENTION

With the prior-art light detection device, the photoelectrons that havepassed through opening 82 of aperture electrode 83 b are focused ontoelectron multiplier 87. In order to make effective use of thephotoelectrons emitted from photocathode 85, the number ofphotoelectrons passing through opening 82 must be made high, and imageforming system 90 and adjustment part 97 are thus required. Also byproviding aperture electrode 83 b, a lens effect is caused by theelectric field formed by photocathode 85 and aperture electrode 83 b.Focusing electrode 83 c is required to correct for this effect. Theprior-art light detection device thus had to be equipped with imageforming system 90, adjustment part 97, focusing electrode 83 c, etc.,and these impeded the making of the device compact.

An object of this invention is to provide a light detection device,which can be made compact while being made low in thermal noise.

This invention's light detection device comprises an optical fiber,having an end face that serves as a light exiting surface, and aphotoelectron emitting part, formed on the end face and emittingphotoelectrons based on light exiting from the end face.

With this invention, since a photoelectron emitting part (for example, aphotocathode) is formed on an end face of an optical fiber, an imageforming system for imaging light onto the photoelectron emitting partand an adjustment part for fine adjustment of the lens of the imageforming system are made unnecessary. Since an aperture electrode is alsomade unnecessary by the same reason, the lens effect, caused by theelectric field formed by the photoelectron emitting part and theaperture electrode, will not occur. Thus by this invention, a focusingelectrode for correcting the lens effect does not have to be disposed.Also, since the photoelectron emitting part is formed on the end face ofthe optical fiber, the photoelectron emitting part can be made compact.Due to the above reasons, a light detection device can be made compactby this invention.

Also, since the photoelectron emitting part can be made compact asdescribed above, the thermal noise can be reduced. The signal-to-noiseratio in measurement can thus be made satisfactory by this invention.

With the present invention, a structure is preferably arranged whereinthe optical fiber includes a core part, at least a part of the end faceincludes the core part, and the photoelectron emitting part is formedjust on the core part of the end face. Since the photoelectron emittingpart can thus be made even more compact, the thermal noise can bereduced and the signal-to-noise ratio in measurement can be madesatisfactory.

With the present invention, a structure is preferably arranged wherein adiffraction grating for wavelength selection is formed on the core part.With this invention, a structure is preferably arranged that includes alight shielding cladding disposed on the surface of the optical fiber inorder to prevent leakage of light from the optical fiber. With thisinvention, a structure is preferably arranged wherein the optical fiberincludes another end face that serves as a light incidence surface andthe light detection device includes an optical fiber connector, which ismounted to the other end face. With this invention, a structure ispreferably arranged that includes a cooling part for lowering thetemperature of the photoelectron emitting part.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of an example of a light detectiondevice of an embodiment.

FIG. 2 is a schematic sectional view of another example of the lightdetection device of the embodiment.

FIG. 3 is a schematic view of a prior-art light detection device.

BEST MODES FOR CARRYING OUT THE INVENTION

A preferred embodiment of this invention shall now be described usingthe drawings. FIG. 1 is a schematic sectional view of an example of alight detection device of this embodiment. A light detection device 1 isequipped with a vacuum container 10, formed of a glass tube, theinterior of which is put into a vacuum condition, and an optical fiber20, comprising a core part 21 and a clad layer 23, formed on theperiphery of core part 21.

Vacuum container 10 has one end face 11 and another end face 13. An endpart 25 of optical fiber 20 is inserted from end face 11 and fixedinside vacuum container 10. At end part 25 is an end face 27 of opticalfiber 20. A light signal L, which has propagated through core part 21from a light source, exits from end face 27. On the core part 21 portionof end face 27 are laminated a substrate metal layer 32, which has beenvapor deposited upon roughening the surface at the nanometer level toenable metal to be adsorbed readily, and a photocathode 30, which is anexample of a photoelectron emitting part. An external photoemissioneffect occurs due to photocathode 30. That is, by the incidence of lightsignal L, exiting from end face 27, onto photocathode 30, photoelectronsare emitted from photocathode 30 into vacuum container 10. As a methodof forming photocathode 30 on end face 27, there is, for example, thefollowing method. That is, first, a metal layer is vapor deposited ontoend face 27. By patterning this metal layer by photolithography andetching, the metal layer is left just on the core part 21 portion of endface 27. This becomes the substrate metal layer 32. By then selectivelyvapor depositing the materials of the photocathode onto substrate metallayer 32, photocathode 30 is formed on end face 27.

Inside vacuum container 10, an electrode 40, which is electricallyconnected to photocathode 30 via substrate metal layer 32, is positionedand also, an electron multiplier 50, is positioned so as to opposephotocathode 30 across a predetermined distance. A known electronmultiplier may be used as electron multiplier 50. The structure andmaterials of electron multiplier 50 are various and since the currentmultiplication factor, time response characteristics, etc., of lightdetection device 1 differ according to these, the structure andmaterials of electron multiplier 50 are selected according to thepurpose of use of light detection device 1. Inside vacuum container 10,a readout electrode 60 is positioned between end face 13 and electronmultiplier 50, and a part of readout electrode 60 is drawn out to theexterior via end face 13. A photomultiplier tube is arranged from vacuumcontainer 10, photocathode 30, and electron multiplier 50.

The operation of light detection device 1 shall now be described. Lightsignal L that has propagated through core part 21 of optical fiber 20 ismade incident on photocathode 30 via end face 27 of optical fiber 20.Electrons inside photocathode 30 are thereby excited and photoelectronsare emitted into the vacuum (external photoemission effect). Thephotoelectrons are made incident on electron multiplier 50.Photoelectrons, which are current-multiplied by secondary electronemission being repeated at electron multiplier 50, are sent to readoutelectrode 60.

With light detection device 1, optical fiber 20, through which lightsignal L flows, is equipped and photocathode 30 is formed on end face 27of optical fiber 20. An image forming system, focusing electrode, etc.,are thus made unnecessary and the device can be made compact. Also,light propagation and photoelectric conversion can be made high inefficiency.

Also with light detection device 1, since photocathode 30 is formed onlyon core part 21 of end face 27, the photocathode can be made compact.Since the thermal noise can thus be reduced to the limit, thesignal-to-noise ratio in measurement can be made satisfactory.Photoelectric surface 30 may also be formed on core part 21 and on cladlayer 23 of end face 27.

The above effects shall now be described specifically using numericalvalues. With light detection device 1, when for example a multi-modefiber with which the diameter of core part 21 is 125 μm is used,photocathode 30 will be 1/1600th that of a photocathode with a diameterof 5 mm (photocathode of a normal size) in area ratio. Also for example,with a prior-art type, with which the photocathode is GaAs and a coolingpart for the photocathode is equipped, the noise level of thephotocathode is approximately 100 cps. With light detection device 1,the thermal noise becomes 0.063 cps.

Another example of the light detection device of the present embodimentshall now be described. FIG. 2 is a schematic sectional view of thislight detection device 3. With regard to light detection device 3, thedifferences with respect to light detection device 1, shown in FIG. 1,shall be described. Of the components making up light detection device3, those that are the same as the components of light detection device 1shall be provided with the same symbols and description thereof shall beomitted.

A diffraction grating 29 is formed on a part of core part 21 of opticalfiber 20. Thus from within a light signal, just the wavelength componentthat is desired to be measured can be selected. Also, a light shieldingcladding 22 is formed on the periphery of optical fiber 20. The leakageof the light signal inside optical fiber 20 to the exterior can therebybe prevented. An FC type optical fiber connector 70 is attached to endpart 24 of optical fiber 20 at the opposite side of end part 25. Thoughphotocathode 30 is formed on just core part 21 of end face 27, it may beformed instead on core part 21 and clad layer 23 of end face 27.

A Peltier cooler 13 is positioned in the vicinity of end face 11 andphotocathode 30 inside vacuum container 10. Peltier cooler 13 has athrough hole and end part 25 of optical fiber 20 is passed through thisthrough hole. Photoelectric surface 30 is cooled by Peltier cooler 13.Thermal noise can thus be reduced. The operation and effects of lightdetection device 3 are the same as those of light detection device 1.

1. A light detection device comprising: an optical fiber, having an endface that serves as a light exiting surface; and a photoelectronemitting part, formed on the end face and emitting photoelectrons basedon light exiting from the end face.
 2. The light detection deviceaccording to claim 1, wherein the optical fiber includes a core part, atleast a part of the end face includes the core part, and thephotoelectron emitting part is formed only on the core part of the endface.
 3. The light detection device according to claim 1, wherein adiffraction grating for wavelength selection is formed in the core part.4. The light detection device according to claim 1, further comprising alight shielding cladding, disposed on the surface of the optical fiberin order to prevent leakage of light from the optical fiber andintrusion of external light into the optical fiber.
 5. The lightdetection device according to claim 1, wherein the optical fiberincludes another end face that serves as a light incidence surface andthe light detection device further comprises an optical fiber connector,which is mounted to the other end face.
 6. The light detection deviceaccording to claim 1, further comprising a cooling part for lowering thetemperature of the photoelectron emitting part.
 7. The light detectiondevice according to claim 1, wherein a metal layer is positioned betweenthe end face and the photoelectron emitting part.
 8. The light detectiondevice according to claim 1, further comprising a light shieldingcladding, disposed on the surface of the optical fiber in order toprevent leakage of light from the optical fiber.